Composites of metal and resin are used in a wide range of industrial fields. Generally, the metal and the resin are joined together by adhesive, and more recently a new method of injection joining for manufacturing composites of metal and resin has been used. In this method, a molten resin material is injected onto a metal piece that has been inserted in advance into an injection molding mold. The metal piece has a surface combined with the resin. Generally, before inserting the metal piece in the mold, a surface of the metal piece is treated by an etchant to form a number of micropores so that a stronger bond is formed with the resin material.